Municipal and industrial waste waters are normally treated in biological treatment plants before they are introduced into a receiving body. The biological treatment plant may consist of several stages, wherein the main stage in most cases incorporates a so-called activated sludge stage. The sewage sludge is separated in a post-treatment stage. Provision may also be made for the waste water to be subjected to sand/gravel filtration. The waste waters treated thus generally attain the required limit values in terms of the concentration of the residues and microorganisms contained in them.
However, there are cases in which the waste water treatment described is insufficient, for instance if greater demands are imposed with regard to germ count, loading with difficultly degradable organic substances, odour and colour. For example, further treatment is required when the waste water treated is to be introduced to bathing water or is to be used for irrigation purposes in agriculture.
It is now common practice to provide for disinfection of the waste water with UV radiation to reduce only the pathogenic microorganisms contained in the waste water (bacteria, viruses, single-celled organisms). However, this disinfection is not suitable for removing difficultly degradable organic substances, odorous substances and dyes. Moreover, the disinfecting capacity of the UV irradiation devices is reduced if the waste water is turbid because some of the radiation is already absorbed in the water itself.
Furthermore, disinfectants with ozone addition are known which also reduce the difficultly degradable substances, odorous substances and dyes mentioned by oxidation. However, a relatively high ozone concentration, e.g. 20 mg of ozone per liter, is required to achieve the reduction of both the difficultly degradable substances and the microorganisms aimed for. This ozone addition is associated with high investment and operating costs.
A third possibility method of effective waste water treatment is membrane filtration, which again is many times more expensive than ozone treatment, in terms of investment and operating costs, under the conditions described above.
Similar situations are also conceivable in the case of other waters, drinking water for example. If difficultly degradable substances such as chlorinated hydrocarbons or aromatic hydrocarbons are contained in the drinking water, in addition to a concentration of microorganisms that requires treatment, low cost effective UV disinfection is insufficient to degrade these substances. In such situations ozone addition has been installed of UV disinfection, giving rise to the higher costs mentioned.
Various methods for treating waste waters with ozone and UV are also known from the following publications:
EP 0696258 shows a water treatment unit in which ultraviolet light is used for the disinfection. Here, the light source is designed so that every very short wave radiation is able to enter the water and ozone is formed there in the immediate vicinity of the UV source. The expected ozone concentrations are low and lie in the mg/m3 range. In air with approximately 40 kWh/kg of ozone, the energy expenditure for ozone production by UV radiation is higher than when generating ozone with electrical discharge, which requires approximately 7 kWh/kg of ozone. Direct ozone generation in water by means of UV has an even lower efficiency than in air. The device is therefore unsuitable from the point of energy expenditure and for use in municipal treatment plants or drinking water supply plants.
A similar device is known from WO 97/36825. Here too, ozone is generated within the concentration range of a few mg/m3 by a photochemical process by means of ultraviolet radiation. As far as ozone production is concerned, this plant cannot be operated economically on the scale of municipal plants either.
A device for oxidising organic constituents in aqueous media is known from the German Offenlegungsschrift DE 2618338. In this plant, the object of the method is the complete oxidation of the organic substances, for which a very high ozone concentration is used, e.g. in the range of 860 g/m3. The aqueous media are treated by ozone in several baths, connected one behind the other, through which the ozone is conducted in the counterflow. In the last bath provision is made, in one embodiment of the invention, for an additional UV irradiation which reinforces the oxidation effect.
The action of the UV light is not used to the optimum degree in this configuration since the UV radiation is absorbed more effectively in the presence of gas bubbles resulting from the introduction of ozone than in a gas-free medium. Moreover, the ozone can be degraded even on a molecular basis in the high concentration indicated by UV light so that it is present in the later stages in a lower concentration than would be the case without UV irradiation. The degradation of ozone by irradiation with UV light in the aqueous phase can be intentionally used to activate ozone (conversion to radicals). These radicals will then effect a faster, further decomposition. However, in the actual waste water, the effect (improved degradation due to radical reactions) is superimposed by secondary reactions. This system is therefore also disadvantageous as far as the optimum action of ozone and UV is concerned, and is unsuitable for economic, large-scale operation.
A device and a method for the ozone treatment of water are known from DE 19509066 A1. No ozone concentration in the water is indicated in this method. At the outlet of the ozone treatment, UV radiators are provided which are designed to degrade any residual content of ozone in the water. Degasification of the treated water takes place in the direction of flow behind the UV irradiation. Here too, the UV irradiation cannot attain its full effect because the water still contains gas bubbles at the time of irradiation.
A method for oxidising organic substances in the water, operating on the basis of concentrations of a few 100 g/m3 of ozone, simultaneous addition of H2O2 and simultaneous irradiation with UV radiators, is known from DE 3884808 T2. Even in these methods, the UV radiation cannot attain the maximum effect because the water is irradiated during the ozone treatment. As mentioned above, this results on the one hand in premature ozone degradation due to the action of radiation and on the other hand in a reduction in UV transmission due to the gas contained in the water in the form of bubbles. Here, the various chemical-physical processes (ozone reactions, activated ozone reaction with H2O2/activated ozone reaction with UV) are used simultaneously, where the efficiency of each individual process is not used to the optimum degree and is even reduced by secondary and cross reactions.
Finally, a method is known from WO 94/11307 in which the waste water is sprayed into a landfill in a treatment chamber under the action of ozone and UV. This method is very expensive because the waste water has to be pumped and sprayed under pressure. The effectiveness, in terms of the efficiency of the energy expended, is relatively small because not only is it expensive to pump and spray the waste water to be treated, but the UV radiation is not used to the optimum degree. Because of the high energy expenditure such a method is not economically suitable for highly contaminated waste waters.